The plywood in my bus was attached with special fluted nails. The nails were attached to the floor pretty well by rust as well. I found that a large chisel and a 6' long pry bar was the key to getting the plywood up in large chunks and sometimes even in one piece. First I would hammer the chisel under the edge, maintain the torsion by inserting some object (bricks, logs, or whatever), and then come in with the pry bar. I'm sure this would work with screws too, since the wood just rips over the head. Then you go in with a claw hammer or crowbar to pull out the nails; or if screws take your pick of vice grips or angle grinder, etc.

Mine had a (large) number of #14 x 1" screws. Once I got those out (or at least spinning), I used a large pry-bar to get them up after I'd taken off all the nicely curved trim where the walls met the floor.

Also, if you have exit doors, check to make sure you've taken off any angle protectors before you start to pry, as they can really hold strongly.

"gobucks" pretty much hit on everything. We just finished tearing out the floor in our '74 Bluebird and the floor was just about all rotten at the front and rear of the bus. We were lucky enough that most of the screws did come out. I used a impact screw gun to remove the screws so that it didn't immediately strip them. One suggestion i do have is that there is a metal trim piece where the front door stairs meet the floor. I recommend saving that piece if at all possible because you will never find another piece bent that way.